Fixes for compute_damage
A few changes:
* Make `compute_damage` short-circuit more, by skipping comparisons if they will only set flags that are already set.
* Update some obsolete comments.
* Add a bunch of missing properties to `compute_damage`.
* Set `RECONSTRUCT_FLOW` when `text-decoration` changes, because it can affect text fragment splitting/merging.
I believe this is all of the computed properties except for the animation properties. (I wasn't sure what to do with those.) After this lands I'd like to work on moving this information into `properties.mako.rs` so that this function can be auto-generated, and so that newly-added properties can default to setting all flags instead of none.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/10109)
<!-- Reviewable:end -->